Add waiting list functionality for leagues

Leagues have a "Team Count" property on them which currently determines what the content of the confirmation email that is sent to the team organisers after they have registered.  However, once they have registered, there isn't really an easy way to see which teams registered first, and which did so after the league was already full.

It would be good to have some form of "Waiting List" for each league, which teams would drop into automatically if they register (and potentially pay...) after a league is already full (ie, there are already x number of teams in the league/season, where x is more than the value specified in the "Team Count" property of the league).

It might also be appropriate to rename the "Team Count" field to "Team Capacity"

The waiting list should be easily ordered by registration date, and it should be easy to move teams from the waiting list to the league, or from the waiting list to ANOTHER league.